Chemical test kits which utilize various calorimetric methods for the determination of any of several different chemical species have long been used. In calorimetric testing, the concentration of a particular chemical substance is determined by measuring the color intensity of the sample. In some cases the color in the sample is due to the species of interest while in most cases one or more chemicals are added to the sample to react with the species of interest so as to develop a color whose intensity is in proportion to the concentration of the species being determined. In some rare cases, color is bleached from samples in proportion to an analyte concentration.
Mathematically, the amount of light passing through a colored material is inversely proportional to the concentration of the colored species and the path length of the light passing through the material When using similar sample cells, the light passing through a sample is inversely proportional to the analyte concentration. In other words, the color of the sample is proportional to the analyte concentration, except for bleaching reactions.
Visual detection of color intensity is limited by several physiological factors. The human eye's color perception is not directly proportional to light intensity and color perception varies considerably between individuals. Even for one person, several factors limit the color perception. Instruments can use a narrow band of light wavelength and quantify the light intensity consistently. The human eye detects all wavelengths simultaneously and perceives a combined signal. Besides the inherent difficulties in basic color perception, differences in shape of objects, gloss, texture, light source and background variations also complicate color differentiation comparison of similar colors.
Several methods have been used to provide reference colors or standards for visual comparison of between unknown samples and references for calorimetric determinations. One of the most common has been to provide colored liquids in sealed glass tubes similar to the tubes used for samples. This method has the advantage of being the most visually similar to samples, but is relatively complicated and expensive to prepare, and the tubes of standards are fragile. If the tubes are dropped, they may break and the color reference is lost, making testing impossible. Complications in production include: (1) actual test chemistry may not produce a suitably stable color, and (2) other materials used to provide the color must provide similar color perception for both hue and intensity in a wide variety of ambient light conditions (dyes commonly have different absorption spectrums than found in chemical tests and may be perceived correctly using the one light source and be obviously different with another light source), and (3) some desirable color dyes and solutions are hazardous.
Other methods of providing reference colors have included: (1) colors printed on a variety of materials, (2) colored transparent glass or plastic pieces, and more recently, (3) color pieces of varying depth which provide the perception of different intensities of the same hue all from one material (e.g., Hach color cubes and color discs). This approach eliminated much of the expense in preparing the reference colors and the fragility problem, but increased difficulties due to change in shape, texture, gloss and backgrounds.
There has not heretofore been provided a color comparator having the features and advantages provided by the present invention.